As demonstrated by the huge number of patents existing in this field, the use of different security elements making the forgery of documents difficult has been extended in recent years. Some of these elements are detectable by human beings; meanwhile other security elements which are incorporated into documents require the use of special tools for detection thereof. These tools include spectroscopic methods such as UV-VIS absorption spectroscopy, fluorescence emission spectroscopy, IR spectroscopy or Raman spectroscopy.
Thus, luminescence pigments or substances have been incorporated into various security documents for certifying the authenticity thereof, the detection or observation of which requires the use of an excitation light in a particular region of wavelengths (for example UV light).
Today, the only fluorescent systems known having huge Stokes' displacement of greater than 200 nm and slow decaying times of more than microsecond are based on rare earth ions. However, they present multiple drawbacks such as: the difficulty in incorporating thereof in matrices such that they do not lose their fluorescent characteristics; the existence of fixed and particular excitation, emission and Stokes' displacement characteristics corresponding to each rare earth, therefore they are not susceptible to being changed, and they are expensive and scarce materials. Examples of these rare earth luminescent systems as security markers are described in documents U.S. Pat. No. 4,598,205, U.S. Pat. No. 4,452,843 and U.S. Pat. No. 4,463,970.
Therefore, there is a clear need to develop new compositions and methods which make forging security documents difficult.